Chlamydia is a pathogenic bacterium with a significant impact on public health. In 2006, more than a million chlamydial infections were reported to the CDC making it the most commonly reported infectious disease. The ability of this organism to cause disease is related to its unusual developmental cycle, which takes place inside a human cell. Our long term objective is to understand how this pathogen controls the expression of its genes at different times in the developmental cycle so that it can successfully grow and replicate. Our central hypothesis is that the three temporal classes of early, mid and late chlamydial genes are coordinately regulated at the transcriptional level by distinct mechanisms. Aim 1 will determine if mid genes, which are the largest temporal class of chlamydial genes, are activated by increased DNA supercoiling. Aim 2 will examine the mechanisms that regulate late genes to prevent their premature expression. Studies will investigate whether the two subsets of late genes are each controlled by a repressor that prevents transcription. Aim 3 will investigate if early genes are selectively expressed at the beginning of an intracellular infection because they are resistant to an inhibitor of RNA polymerase. Successful completion of these studies will help us to understand how Chlamydia controls the programmed expression of its genes. These findings may lead to novel therapeutic strategies for treating chlamydial infections by interrupting the developmental cycle.